In such areas where there are many radio broadcasting stations or during the night where many broadcasting signals of foreign countries can be received, interference by adjacent disturbing signals is easily generated and thereby the radio program of a desired station cannot be received in good receiving condition. It is necessary to make narrower the pass-band width of an IF filter provided between a tuner and intermediate frequency amplifier in order to receive a broadcasting signal from the desired station without interference by the adjacent disturbing signal, but the pass-band width of IF filter is restricted by the maximum modulation frequency and the maximum degree of modulation. If the pass-band width is narrowed ignoring such restriction, the distortion coefficient is deteriorated and the broadcasting signal cannot be received in good receiving condition. Therefore, in the radio receiver of the prior art, at least two IF filters having different pass-band widths are provided between the tuner and IF amplifier. If the broadcasting signal is disturbed by the adjacent disturbing signal, an IF filter having a narrow pass-band width is selected by the switch in order to eliminate interference by the adjacent disturbing signal. When the receiving signal is not interfered by the adjacent disturbing signal, an IF filter having a wider pass-band width is selected by the switch.
In the case of a radio receiver to be mounted on a vehicle (car radio), the conditions of signals to be received are changing from time to time. Therefore, the car radio receiver of the prior art is required to adjust the pass-band width of the IF filter with the switch, etc. in accordance with the receiving conditions and moreover fine control of pass-band width requires many IF filters and the switching circuit. Thereby, constitution is complicated and conditions of interference by the adjacent disturbing signal cannot be obtained accurately, thus disabling reception of radio program in good receiving condition.